Use of Page Manager is optional, as each web page will inherit the settings for its Page Type.  Page Types by default inherit the Site Settings.  Thus it is only necessary to use Page Manager when you wish to override the Site Settings.

You can configure the default settings for a specific Page Type using Page Type Manager.  Page Type Manager is explained in the chapter of this guide titled Utilities Features Menu in Detail.  E.g. if you want all of your Product Detail pages to be the same, use Page Type Manager to select Product Detail Page and then edit the settings there. When you add a new Product Item, it will inherit the Product Detail Page Type settings.  If you want to override them for a specific Product Item, you can then select the Page Manager option in Item Management to do so.

The Preview option located on the Page Manager page allows you to view how a Collection will appear.  So if you want to experiment, create a new collection and use the Preview option to view it.   This will not affect your live website.